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RVCC Fall 2008 CHEM 103 – General Chemistry I. Chapter 9: Molecular Structures. Chemistry: The Molecular Science, 3 rd Ed. by Moore, Stanitski, and Jurs. Molecular Structure. Molecular geometry is the general shape of a molecule or the arrangement of atoms in three dimensional space.

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slide1
RVCC Fall 2008

CHEM 103 – General Chemistry I

Chapter 9:Molecular Structures

Chemistry: The Molecular Science, 3rd Ed.

by Moore, Stanitski, and Jurs

molecular structure
Molecular Structure

Molecular geometry is the general shape of a molecule or the arrangement of atoms in three dimensional space.

Physical and chemical properties depend on the geometry of a molecule.

molecular structures
Molecular Structures

3-D Model

3-D Drawing

does it matter the thalidomide story
Does it matter?The Thalidomide Story

The chemical structure of thalidomide.

models – enantiomers (mirror image)

“The Same and Not the Same”, by Roald Hoffmann

1995, Columbia University Press

slide5
Does it matter?Fatty Acids

‘trans’ fatty acid

‘cis’ fatty acid

vsepr model
VSEPR Model

The Valence Shell Electron Pair Repulsion model predicts the shapes of molecules and ions by assuming that the valence shell electron pairs are arranged as far from one another as possible to minimize the repulsion between them.

vsepr model7
VSEPR Model

Electron Pair Geometry –

is determined by the number and arrangement of all electron pairs (bonding and lone) around the central atom.

Molecular geometry–

is determined by the arrangement of atoms (or bonding electron pairs only) around the central atom.

H

H

H

N

:

In molecules with no lone pairs,

Electron Pair Geometry= Molecular Geometry

slide8
Fig. 9-4, p.383
  • AXE shorthand notation:
      • A - central atom
      • X - terminal atoms
      • E - lone pair electrons

AX3E0

predicting molecular geometry vsepr
Predicting Molecular Geometry: VSEPR
  • Only five basic shapes.
  • When a lone pair replaces an atom, the molecular geometry changes as well as the angles.

# e- pairs 2 3 4 5 6

Fig. 9-4, p.383

predicting molecular geometry vsepr10
Predicting Molecular Geometry: VSEPR
  • Draw the Lewis structure.
  • Determine how many electron pairs (bonded and non-bonded) are around the central atom. **Treat a multiple bond like a single bond when determining a shape.
  • Write the AXE shorthand notation.
  • Determine the electron pair geometry (**one of the five basic shapes).
  • If the molecule has lone pairs around the central atom, then determine the molecular geometry. (This is a subset of the electron geometry.)
linear electron geometry two e pairs about central atom
..

..

Linear (Electron Geometry)Two e- pairs about central atom

bond lone Molecular

pairs pairsGeometry

2 0 linear

1 1-3 linear

The molecular geometry here is the same as the electronic

geometry even though there is a lone pair. ‘Two points make a line.’

slide12
Cl Be Cl

Predicting Molecular Geometry

Example 1:BeCl2

AX2E0

1. Draw the Lewis structure

2. Two electron pairs around the central atom.

Two bonded and Zero lone pairs.

  • electron pair geometry = molecular geometry
  • Geometry is Linear. Bond angle is 180o.
slide13
..

..

..

..

..

..

Trigonal Planar (Electron Geometry)Three e- pairs about central atom

bond lone Molecular

pairs pairs Geometry

Model

3 0 triangular planar

2 1 angular (bent)

1 2 linear

predicting molecular geometry
:F:

B

..

..

F:

:F

..

..

Predicting Molecular Geometry

Example 2: BF3

..

AX3E0

Three electron pairs around the central atom.

Three bonded and Zero lone pairs.

triangular planar

(or trigonal planar)

predicting molecular geometry15
O S O

S

O

O

Predicting Molecular Geometry

Example 3:

SO2

AX3E0AX2E1

Three electron pairs around the central atom.

Two bonded and One lone pairs.

electron geometry = triangular planar.

molecular geometry = bent or angular.

slide16
..

..

..

..

..

..

Tetrahedral (Electron Geometry)Four e- pairs about central atom

bond lone

pairs pairs

4 0 tetrahedral

Model

3 1 triangular pyramidal

2 2 angular (bent)

predicting molecular geometry17
H

C

H

H

H

Predicting Molecular Geometry

Example 4:

AX4E0

CH4

Four electron pairs around the central atom. Zero lone pairs.

tetrahedral

electron pair geometry = molecular geometry

predicting molecular geometry18
H N H

H

N

H

H

H

Predicting Molecular Geometry

Example 5:

AX4E0 AX3E1

NH3

Four electron pairs around the central atom.

Three bonded and One lone pair.

electron geometry = tetrahedral.

molecular geometry = triangular pyramidal

predicting molecular geometry19
H O HPredicting Molecular Geometry

Example 6:

AX4E0 AX2E2

H2O

Four electron pairs around the central atom.

Two bonded and Two lone pairs.

O

H H

electron geometry = tetrahedral

molecular geometry = angular or bent

predicting molecular geometry20
Predicting Molecular Geometry

Tetrahedral - bond angles

Order of increasing repulsion:

bonding pair-bonding pair < bonding pair-lone pair < lone pair-lone pair

slide21
90°

..

..

..

..

..

..

..

120°

Seesaw

T-shaped

Linear

Triangular bipyramidal

Trigonal Bipyramidal (Electron Geometry)Five e- pairs about central atom

The atoms are non-equivalent.

Green atoms are axial;

blue atoms are equatorial.

**Put lone pairs

in the equatorial

positions.

predicting molecular geometry22
:

:

:

F :

F :

: F

:

:

:

:

: F :

P

: F :

:

Predicting Molecular Geometry

Example 7: PF5

AX5E0

Five electron pairs around the central atom.

Zero lone pairs.

electron and molecular geometry=

trigonal bipyramidal

predicting molecular geometry23
:

:

F

:

:

F

:

:

S

:

:

F

:

:

:

F

:

Predicting Molecular Geometry

Example 8: SF4

AX5E0 AX4E1

Five electron pairs around the central atom.

Four bonded and One lone pair.

electron geometry = trigonal bipyramidal

molecular geometry = seesaw

predicting molecular geometry24
:

:

:

F

:

:

:

F

Br

:

:

:

:

F

:

Predicting Molecular Geometry

Example 9: BrF3

AX5E0 AX3E2

Five electron pairs around the central atom.

Three bonded and Two lone pairs.

electron geometry = trigonal bipyramidal

molecular geometry = T-shaped

predicting molecular geometry25
:

:

F

:

:

Xe

:

:

:

F

:

:

Predicting Molecular Geometry

Example 10: XeF2

AX5E0 AX2E3

Five electron pairs around the central atom.

Two bonded and Three lone pairs.

electron geometry = trigonal bipyramidal

molecular geometry = linear

slide26
..

..

..

90°

Square planar

Square pyramid

Octahedral

Octahedral (Electron Geometry)Six e- pairs about central atom

Equivalent atoms

predicting molecular geometry27
S

:F

:

:

:

:

:

:F:

F:

F:

:

:F

:F:

:

:

:

:

Predicting Molecular Geometry

Example 11: SF6

AX6E0

Six electron pairs around the central atom.

Six bonded and Zero lone pairs.

electron geometry = octahedral

molecular geometry = octahedral

predicting molecular geometry28
:

:

:

F

:

:

:

:

F

F

I

:

:

:

:

:

:

:

F

F

:

:

Predicting Molecular Geometry

Example 12: IF5

AX6E0 AX5E1

Six electron pairs around the central atom.

Five bonded and Two lone pairs.

electron geometry = octahedral

molecular geometry = square pyramidal

predicting molecular geometry29
:

:

:

F

:

F

:

:

:

Xe

:

:

F

F

:

:

:

:

:

Predicting Molecular Geometry

Example 13: XeF4

AX6E0 AX5E1

Six electron pairs around the central atom.

Four bonded and Two lone pairs.

electron geometry = octahedral

molecular geometry = square planar

practice
Practice
  • CO2
  • SO2
  • ClO2-
  • ICl
  • ICl3
  • ICl5
  • GeF4
  • SeF4
  • XeF4
bond angles
Bond Angles

CHO

Give the approximate values for the indicated

bond angles.

COH

OCN

HNH

molecular geometry dipole moment and polarity
Molecular Geometry Dipole Moment and Polarity

Electronegativity (EN) values are used to predict the polarity of covalent bonds. The greater EN, the more polar will be the bond. A polar bond has a dipole or slight separation of charge (from the unequal sharing of bond electrons). [Chapter 8]

The polarity of a molecule depends on the sum of all the bond dipoles (vectors). If there is a net dipole for the molecule, than the molecule is polar. A molecule that has polar bonds may or may not be polar.

The dipole moment (μ) is a measure of the degree of charge separation or the polarity.

slide35
O

H H

Molecular Geometry Dipole Moment and Polarity

d+

d-

d-

nonpolar, bp=-79C

dipole moment, μ = 0 D

d-

..

..

+

Net

dipole

d+

d+

polar, bp=100C

dipole moment, μ = 1.85 D

slide36
Molecular Geometry Dipole Moment and Polarity

In general, a molecule is polar if:

  • it isn’t a basic VSEPR shape (symmetrical)

Ex:H2O, bent(polar)

  • or if the terminal atoms/groups in a

basic VSEPR shape differ.

Ex:CH2Cl2, tetrahedral (polar)

dipole moment and molecular geometry
Dipole Moment and Molecular Geometry

Molecules that exhibit any asymmetry in the distribution of electrons would have a nonzero net dipole moment. These molecules are considered polar.

Non polar

VSEPR shape

identical atoms

Polar

VSEPR shape

atoms differ

slide39
PF4Cl

PF3Cl2

PF3Cl2

PF5

Molecular Geometry Dipole Moment and Polarity

Non polar

Atoms differ. BUT can be divided into nonpolar VSEPR shapes:

linear + triangular planar

+

+

Non polar

VSEPR shape

identical atoms

Polar

Atoms differ. Doesn’t divide into nonpolar VSEPR shapes

Polar

VSEPR shape

atoms differ

slide40
F

F

S

:

F

F

..

F

F

Xe

..

F

F

Dipole Moment and Molecular Geometry

SF4

F

ClF3

:

SeeSaw

No symmetry → polar

Cl

:

F

T-shaped

No symmetry → polar

F

F

XeF2

:

XeF4

Xe

:

:

Linear

Symmetric → non polar

F

Square Planar

Symmetric → non polar

slide41
Molecular Geometry Dipole Moment and Polarity
  • Which compound is the most polar?
  • Which compounds on the list are non-polar?

CO, PCl3, BCl3, GeH4, CF4

orbitals consistent with molecular shape
Orbitals Consistent with Molecular Shape

Lewis dot + VSEPR gives the correct shape for a molecule. BUT…

How do atomic orbitals (s, p, d …) produce these shapes?

Valence bond theorydescribes a bond as an overlap of atomic (hybrid) orbitals.

valence bond model
H

1s

F

2s

2px

2py

2pz

Valence Bond Model

H2

HF

valence bond theory
Be

[He] 2s2

2s

C

[He]2s22p2

2s

2px

2py

2pz

Valence Bond Theory

…and, why do we draw

the Lewis structures like we do?

This works for H2 and HF, but…

  • Why does Be form compounds?
    • no unpaired electrons
  • Why does C form 4 equivalent bonds at tetrahedral angles?
    • only two unpaired electrons
    • p orbitals are at 90° to each other (not 109.5°)
orbitals consistent with molecular shapes
Orbitals Consistent with Molecular Shapes
  • Atomic orbitals (AOs) can be hybridized (mixed).
  • Sets of identical hybrid orbitals form identical bonds.
  • # AOs that hybridize = # hybrids orbitals.
  • s + p sp + sp
  • s + p + p sp2 + sp2 + sp2
  • etc….
sp hybrid orbitals
2p 2p 2p

Two unhybridized

p orbitals

2p 2p 2p

Energy, E

Promotion

Orbital

hybridization

Two sp hybrid

orbitals on Be in BeF2

2s

Isolated Be atom

2s

sp Hybrid Orbitals

AX2E0,Ex:BeCl2,

sp hybridization occurs around the central atom whenever there are two regions of high e- density.

Two equivalent covalent bonds form (180° apart) LINEAR.

slide48
sp2 Hybrid Orbitals

AX3E0,Ex:BF3

The result is THREE equivalent

hybrid orbitals, in a VSEPR basic shape of trigonal planar.

p. 396

slide49
sp3 Hybrid Orbitals

AX4E0,Ex:CH4

TETRAHEDRAL

slide50
sp3 Hybrid Orbitals

AX3E1 (NH3) and AX2E3 (H2O)

orbitals consistent with molecular shapes51
Cl

Cl

:

:

:

P

:

:

Cl

:

:

:

:

:

Cl

: :

: :

Cl

:

Orbitals Consistent with Molecular Shapes

Describe bonding in PCl5 using hybrid orbitals.

AX5E0

trigonal bipyramidal

We need 5 orbitals.

slide52
sp3d Hybrid Orbitals

3d

valence

shell

3p

hybridization

five equal sp3d hybrid orbitals

X

3s

P atom (ground state)

slide53
3d

sp3d

P atom (hybridized state)

sp3d Hybrid Orbitals

orbitals consistent with molecular shapes54
F

F

F

S

:

:

:

:

:

:

:

:

F

F

:

: :

:

:

:

:

: :

F

:

Orbitals Consistent with Molecular Shapes

Describe the bonding in SF6 using hybrid orbitals.

AX6E0

Octahedral

We need 6 orbitals.

slide55
sp3d 2 Hybrid Orbitals

3d

X

hybridization

3p

six equal sp3d2 hybrid orbitals

X

3s

S atom (ground state)

slide56
3d

sp3d2

S atom (hybridized state)

sp3d 2 Hybrid Orbitals

summary hybrid orbitals
Hybrid Orbital

Geometric Arrangements

Number of Orbitals

Example

sp

Linear

2

Be in BeF2

sp2

Trigonal planar

3

B in BF3

sp3

Tetrahedral

4

C in CH4

sp3d

Trigonal bipyramidal

5

P in PCl5

sp3d2

Octahedral

6

S in SF6

Summary - Hybrid Orbitals
hybridization
Hybridization

Mixed Hybrids (#) Remaining Geometry

s+p sp (2) p+p Linear

s+p+psp2 (3)p Triangular planar

s+p+p+p sp3 (4) Tetrahedral

Mixed Hybrids (#) Remaining Geometry

s+p+p+p+dsp3d (5) d+d+d+dTriangular bipyramid

s+p+p+p+d+dsp3d2(6) d+d+dOctahedral

practice59
Practice

What are the hybridization and approximate bond angles for each C, N, O in the given molecules?

what about multiple bonding
What about… multiple bonding!

According to valence bond theory hybrid orbitals include:

  • single bonds
  • lone pairs
  • one of the bonds in a multiple bond.

The electrons in the unhybridized atomic orbitals are used to form the additional multiple bonds.

multiple bonding
Multiple Bonding
  • A s (sigma) bond is an overlap of orbitals (hybrids) along the bond axis.
  • A p (pi) bond is a overlap of parallel “p” orbitals, creating an electron distribution above and below the bond axis.
multiple bonding62
2s

1s

1s

Multiple Bonding

(unhybridized)

2p

2p

sp2

Energy

(3 sp2 hybrid

+ 1 unhybridized p)

C atom (ground state)

multiple bonding63
1s

1s

Multiple Bonding

(unhybridized)

2p

2p

sp2

2s

Energy

(3 sp2 hybrid +

1 unhybridized p)

O atom (ground state)

practice67
Practice
  • Identify the pi and sigma bonds in the given molecules.

σ

σ

π

σ

σ

σ

π, π

σ

types of intermolecular forces
δ-

δ+

δ+

δ-

Typesof Intermolecular Forces

Intermolecular Interactions

London Forces

Dipole-Dipole Forces

Hydrogen Bonding

(0.05 – 40 kJ/mol)

(5 – 25 kJ/mol)

(10 – 40 kJ/mol)

(Intramolecular Covalent Bond

150 – 1000 kJ/mol)

types of intermolecular forces69
Types of Intermolecular Forces

London Forces

(dispersion forces)

When electrons are momentarily unevenly distributed in the molecule, polarization occurs.

Induced Dipole

All molecules, EVEN nonpolar ones experience London Forces!

(Nonpolar molecules do not experience any other intermolecular interaction)

types of intermolecular forces70
Types of Intermolecular Forces

To boil (l g), molecules must have enough energy to overcome their intermolecular forces.

The higher the intermolecular force …the higher the boiling point!

Dispersion Forces increase with increased number of electrons.

increased

polarizability

types of intermolecular forces71
Types of Intermolecular Forces

A polar molecule is a Permanent Dipole that creates ….

Dipole-Dipole forces

types of intermolecular forces72
Types of Intermolecular Forces

The more polar the molecule (at a given size) …

… the higher the boiling point!

types of intermolecular forces73
Types of Intermolecular Forces

Hydrogen bond

…is established by the attraction between hydrogen and an electron pair on a small, very electronegative atom.

X—H - - -:Z—

X = N, O, F Z = N, O, F

This bond is responsible of determining the three dimensional structure of large proteins molecules

types of intermolecular forces74
Types of Intermolecular Forces

Water:

One molecule can participate in four H bonds with other molecules.

Because of the hydrogen bond, water has a boiling point 200 C higher than if the bond were not present.

practice75
Practice
  • Explain the following boiling points,
    • HF (20˚C), HCl (-80˚C), HBr (-60˚C), HI (-25˚C)
  • Which of the following will form H-bonds:
    • CH2Br2, CH3OCH2CH3, CH3CH2OH, H2NCH2COOH
  • What types of forces must be overcome in these changes?
    • The sublimation of solid C10H8
    • The decomposition of water into H2 and O2
    • The evaporation of PCl3
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